1. Field of the Invention:
The invention relates to an integrated circuit having integral heating elements, temperature sensitive circuitry and thin film resistors therein, and also to methods and apparatus for compensating thermal drift of such integrated circuits.
2. Description of the Prior Art:
Thermal sensitivity, in the form of thermal drift of various electrical parameters, is a major limitation to electronic circuit performance. During integrated circuit manufacturing operations, individual units must frequently be rejected because thermal drift of one or more electrical parameters exceeds specifications. However, many types of electronic circuits are manufactured with adjustable elements, such as thin film resistors, which may be laser "trimmed" to compensate thermal drift of temperature sensitive circuitry. An example of temperature sensitive circuitry which may be compensated for thermal drift by trimming of resistors is a differential amplifier. Thermal drift of the input offset voltage of a differential amplifier occurs if the input transistors are not perfectly matched, but the thermal drift may be compensated by trimming the collector and emitter resistors of the differential amplifier. Other types of circuits in which thermal drift may be compensated by trimming resistors include voltage and current regulators used as reference elements in digital-to-analog converters, analog multipliers, voltage-to-frequency converters, and many types of AC and DC amplifiers. For a more detailed discussion of the analysis and compensation of thermal drift in semiconductor circuits, see "Operational Amplifiers-Design and Application", McGraw Hill Book Co., 1971, edited by Tobey, Huelsman, and myself.
Trimming of thin film resistors to compensate thermal drift of temperature-sensitive parameters has, up to now, involved individual measurements of packaged semiconductor circuits mounted and electrically tested in temperature controlled chambers, referred to as "ovens". The circuits are packaged, other than in wafer form, as they may be mounted in sockets in the oven. This is necessary so that electrical contact can be made for powering and testing the circuit. The packages are of a type having an open cavity to expose the semiconductor chip to a laser beam during laser trimming to compensate the thermal drift. After the laser trimming operation a lid is mounted on the package. The measured data must be stored for later utilization in conjunction with the trimming operation. After the temperature sensitive parameter is measured over a suitable range of temperatures, the circuits are removed from the oven. Ordinarily, a cooling period must be allowed. The devices under test are then mounted in a socket such that the thin film compensation resistors may be controllably positioned in the path of the laser beam. Trimming of the thin film compensation resistor then occurs as the relative positions of the laser beam and the thin film compensation resistor are precisely varied either manually or in response to computer generated control signals produced in response to the above-mentioned stored measured data. Once the proper trimming is achieved, reinsertion of the integrated circuit in the oven and thermal testing confirming the compensation of the thermal drift are required.
The above-described thermal testing, laser trimming, and retesting operations represent a very significant product expense that limts the economies otherwise realizable in the hybrid and monolithic integrated circuit technology. The costs of the ovens, sockets and personality boards for various packages, the cost of storing the measured data while the circuits under test are being transferred to the laser trimming apparatus, the costs and loss of time involved in the necessary manual handling, plugging and unplugging of devices from the sockets of the oven and laser trimming apparatus, and the time required for thermal stabilization at each temperature in the oven before each measurement of the temperature sensitive parameter, all represent costs that could be avoided if compensation for thermal drift could be accomplished while the circuits are still in wafer form.
The general state of the art is indicated in IBM Technical Disclosure Bulletin, Volume 14, No. 6, November 1971, Page 1770, and U.S. Pat. Nos. 3,614,480; 3,842,346; 3,936,789; 3,703,651; 3,465,427; 3,395,265; 3,369,207; 3,289,046; 3,039,604; and 2,743,420.